Raw natural gas pumped of discharged from a well is not suitable for use by consumers because of the impurities that it contains. Various commercial processes have been developed for treating and separating impurities and other undesirable constituents from raw natural gas streams in order to produce consumer gas. Raw natural gas usually contains some level of mercury requiring removal treatment. Since mercury left on processing equipment affects downstream processing, it is highly desirable to remove mercury from the natural gas stream at an early stage. As used herein, the terms consumer gas and sales gas means natural gas that is substantially free of undesirable constituents.
U.S. Pat. No. 4,073,734 to Lowrie, entitled “Marine Separator” discloses a single horizontal separator of produced fluids from oil wells that are mainly water, oil and gas. The separator is used for oil wells and includes many internal conduits and adjustable extensions. These complicated structures make it difficult to separate the mercury and liquid hydrocarbons from the gas. Furthermore, in the natural gas application, little liquid will be formed upon chilling the natural gas to 20 F. Lowrie does not provide separators for removing mercury.
U.S. Pat. No. 4,968,332 to Maher, entitled “Separator Unit”, discloses a separation of oil and salt water from natural gas at a well head. The separator unit prevents oil and/or salt water from discharging into the surrounding natural environment of high pressure, high velocity of stream during venting of a natural gas well head. Thus, even if mercury exists in the natural gas, it will not condense at the well head temperature and it will be present in the gas phase. It may also exist in an organic form dissolved in the liquid hydrocarbon, which can not be separated by gravity.
U.S. Pat. No. 6,409,808 to Chamberlain, et al., entitled “Separators”, discloses a gravity separator for separating oil, water, and gas from raw oil using gravity to form vertically discrete oil and water layers. A cyclone is used to break the emulsion from the crude oil and thereby separate water and gases from the oil. Chamberlain, et al. does not provide a way to separate mercury from gas.
U.S. Pat. No. 2,701,620 to Crawford, entitled “Oil, Gas, and Water Separator”, is directed to a separator for removing excess free water from the oil, gas and water as produced from the well. The separator maintains the desired pressure on the oil or water so that it may be discharged at any elevation within the range of pressure applied to the fluid in the vessel. Thus, the separator removes sludge and emulsion present in the crude oil. The separator has a complex structure that is required to separate sludge and emulsion, but would be ineffective for separating mercury.
U.S. Pat. No. 2,710,071 to Kinser, et al., entitled “Horizontal Separator for Mixed Fluids” discloses a horizontal separator for separating mixtures of gas, oil and water discharged from a producing well. The separator has upper and lower vessels disposed horizontally. However, this separator cannot be used for separating mercury, because mercury will accumulate throughout the vessel and the scrubbers will aggravate the problem of mercury contamination, which will make its cleaning and removal from the vessel a challenge. In addition, the mercury level in natural gas is typically not high enough to travel from the top to the lower portion and then to the sump.
U.S. Pat. No. 2,984,360 to Smith, et al., entitled “Multipurpose Separator”, discloses a separator that can be used with any liquids and gas. However, this apparatus can not be used for mercury. Mercury is toxic and has a health and safety impact on people entering the vessel during shutdowns for maintenance or inspections. If the multipurpose separator were to be used for separating mercury, the mercury-contaminated area would extend to the entire lower portion of the vessel, which would require cleaning before entry of personnel to inspect or perform maintenance activities, thereby rendering it unpractical and expensive to operate.
U.S. Pat. No. 4,982,050 to Gammie, et al., entitled “Natural Gas Treating System Including Mercury Trap”, discloses a method and apparatus for removing mercury that is present in natural gas. The mercury traps described are to be used when mercury removal units have been used upstream at the inlet of the plant, which indicates a very low mercury content in the gas afterward. The mercury removal units can remove up to 0.01 micrograms per cubic meter of mercury from natural gas, which makes further condensation of mercury impossible. In addition, the traps will remove all condensed materials, including liquid hydrocarbons and mercury. It is not economically practical or environmentally acceptable to remove and discard liquid hydrocarbons with the mercury as a waste stream.
There is, therefore, a need in the art for an efficient and cost effective method and apparatus for selectively separating mercury from a natural gas stream.
There is also a need for a continuous separation method and apparatus that can remove mercury that may be present at any level or concentration in a natural gas stream.